This invention relates to a variable ratio gear system, in particular for use in a wind energy conversion system.
Wind energy can be converted into useful forms, such as electricity, by a wind energy converter that includes a rotor, e.g., a low speed propeller. The speed of the rotor is often influenced by local wind conditions and therefore fluctuates arbitrarily. Thus, conventional wind energy converters are typically operated with variable rotor speed and active torque control of the drive train to maintain a more constant generator rotational speed. The mass inertia of the rotor usually allows damping of the speed variations in the drive train, while the active torque control reduces the loads in the drive train and improves the quality of the electrical power fed to the grid. The active torque control also improves the aerodynamic efficiency during partially-loaded operational stage.
To supply electric power to the grid by converting wind energy, it is advantageous to use a constant speed generator (e.g., a synchronous generator directly connected to the grid for power transmission). In addition to economic benefits, the quality of the electric power fed into the grid is usually higher without a power converter, which can be expensive. Furthermore, the constant speed generator can be operated at a medium voltage level so that no additional transformer is needed.
Using a constant speed generator requires maintaining the input shaft to the generator at a constant speed while the input rotor speed changes over time. Therefore, a variable ratio gear is needed in the drive train. Previous approaches for providing a variable ratio gear include using a gear box consisting of combinations of epicyclic gears with a reaction path to control the variable speed ratio. In addition, hydrostatic circuits are used in the control path, since axial piston pumps of good efficiency are commercially available and provide the capability of damping high frequency torque oscillations in the drive train.